Recently, the implantation of sophisticated electronic devices has become a routine occurrence in many therapeutic procedures. Today many forms of medical electrode leads are available which are provided with a metallic distal electrode tip which is placed adjacent excitable tissue, such as the inside wall of the heart, whereby electric currents supplied to the distal tip stimulate the muscle in contact with the tip to initiate or to regulate heartbeats.
There are a number of conflicting design factors which must be resolved when determining the geometric shape of such medical electrode leads. For example, it is desirable to minimize the size of the electrode lead, and particularly the cross-sectional area, to facilitate the passage of the lead through tight places, such as the inside of small blood vessels, in order to reach desired locations. This aspect is particularly important where two leads are inserted through the same vein. Nevertheless, it is vitally important that the electrode lead have an effective anchoring configuration so that once the electrode tip is correctly positioned it remains in that position. Also, providing a smooth surface to minimize insertion and removal resistance lessens the danger of tissue damage and makes accurate location of the electrode tip possible.
As a solution to the need for an anchoring means, it has been proposed to form electrode tips including flanges or tines extending therefrom in order to engage body tissue such as the trabecular muscles inside the heart following insertion of the electrode tip. However, in striking a compromise between the need for anchoring and the need to minimize the resistance to insertion and removal of the electrode tip, some devices have exhibited an unsatisfactory anchoring effect because the size of the flange is restricted by the inside diameter of the smallest vein through which the flange must pass.
The devices that have employed flexible tines usually extend the tines at an acute angle with respect to the longitudinal axis of the electrode lead at a point adjacent the electrode tip. Also, certain prior art devices have employed means to hold the tines against the electrode lead during insertion in an attempt to reduce the resistance to passage through blood vessels. The tines are then released when the distal tip is properly positioned in the heart.
A primary disadvantage of such prior art arrangements stems from the abrupt transitions created at the base of the tines when they are in a folded configuration. Such abrupt transitions are a source of trauma to the inside walls of blood vessels, cause increased resistance to passage of the electrode lead through the vessels, and hence limit the minimum size blood vessel through which the electrode leads may pass.
A new approach to the designing of an anchoring means is set forth in U.S. patent application Ser. No. 114,950 filed Jan. 23, 1980, on behalf of Carl Doring and entitled "Trailing Tine Electrode Lead." The trailing tine electrode lead described therein comprises an exposed conductive distal tip, a conductive shank supporting the distal tip, an electrical conductor coupled to a proximal end of the shank, and an insulating covering over the shank and the conductor. The insulating cover includes a transitional section having a proximal end, and a plurality of flexible tines connected to the insulating cover at and trailing behind the proximal end of the transitional section. The tines are flexible and fold backward along sides of the conductor of the electrode lead upon encountering an obstacle, such as the inside wall of a vein, during insertion. When folded, the tines present a minimized cross-sectional area equal to the sum of the cross-sectional area of the tines and the cross-sectional area of the insulated conductor.
Despite the marked advantages of the trailing tine design, it would be desirable to further minimize the cross-sectional area of the electrode lead in order to facilitate further the insertion and removal of the lead and to minimize trauma to blood vessels as the lead passes therethrough.